1. Field of the Invention
In general, the present invention applies to radar systems. More specifically it relates to the automatic frequency control (AFC) of a variable frequency oscillator within a radar system.
2. Description of the Related Art
A radar system transmits and receives radio frequency (RF) energy. This high frequency energy is difficult to filter and amplify. To facilitate filtering and amplification, the RF echoes from targets are converted to an intermediate frequency (IF) at which filtering and amplification can be more readily accomplished. Conversion is accomplished by mixing the RF signal with a signal obtained from a variable frequency oscillator.
The IF filters and amplifiers have a limited bandwidth. If the oscillator's frequency drifts too much, the IF signal begins to drift outside of the bandwidth of the IF filters and amplifiers, and thereby degrades the radar's performance.
In the past, AFC was used to minimize the frequency drift of the oscillator. This was accomplished by sampling the frequency of the IF signal to verify that it was in middle of the IF bandwidth. If the IF signal began to drift outside of this bandwidth, the frequency of the oscillator was modified to make a correction. In the past, the IF signal was sampled during the RF transmission of the radar's magnetron.
When the magnetron fires, a large amount of energy is transmitted from the antenna into the atmosphere. Ideally all this energy is passed into the atmosphere, however, some energy leaks into the receiver. The leakage energy is mixed with a signal from the oscillator to produce an IF signal that is sampled to adjust the oscillator's frequency. If too much energy leaks into the receiver, there is a temporary shift in the frequency of the oscillator. This shift lasts for the duration of the RF transmission, and results in the oscillator operating at one frequency during RF transmission, and at another frequency during the radar's receive time. Since the IF frequency sample is taken during RF transmission, it includes the temporary shift in the oscillator's frequency. By adjusting the oscillator's frequency based on this inaccurate intermediate frequency sample, the performance of the radar is degraded.
In the past, this problem was addressed by providing as much isolation as possible between the magnetron and oscillator. Unfortunately, this isolation is expensive, takes up a great deal of space, and has a performance that degrades as temperature decreases.